back to stock rom

I want to revert to stock rom (4.1.2) from cm10.1.3. I have a backup of stock rom but don't know the procedure of reverting. Does it requires flashing kernal also and if any risk is involved? plz suggest

Start to read the sticky tutorials how to install a stock rom, how to restore backups etc.
The only risk involved: errors as a result of laziness.
In short: backup your CM, restore the old from recovery. If that doesn't work install the kernel/recovery you used when generating the backup, reboot into recovery and try to restore again.

Just to add to what ThaiDai said, I dont remember if the 10.1 CM recovery uses preload and if it was removed in 10.2.. But when you boot into recovery and if you cant format the preload partition in mounts and storage, then flash any Philz kernel, reboot into recovery, enable preload in settings and restore the backup.

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